I've been trying to solve this issue for 2 days now and now that I've posted the question I found a solution :$
stackoverflow.com/a/73557076/12924241 Inspired me to change the bindingRedirect to the correct version (6.0.1.0).
This is the correct App.config with binding recdirects:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<!--
This binding redirect is needed to make anything that involves calling System.Text.Json to work.
For some reason this is only needed in unit test projects.
-->
<dependentAssembly>
<assemblyIdentity name="System.Runtime.CompilerServices.Unsafe" publicKeyToken="b03f5f7f11d50a3a" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-6.0.1.0" newVersion="6.0.1.0" />
</dependentAssembly>
</assemblyBinding>
</runtime>
</configuration>